In recent years, air bags have been widely used as the safety devices for protecting the passengers upon collision. As to a base fabric for an air bag, a base fabric prepared by coating surface of woven fabric with silicone rubber or the like has been used. However, an air bag using such a surface-coated woven fabric causes a considerable increase in cost as a result of a coating treatment and also becomes heavy and thick. Therefore, recently, investigations for an air bag using a non-coat woven fabric where no surface coating is done or the so-called non-coat air bag have been carried out and a part thereof has been already in practical use.
Further, in recent years, there has been a demand for making an air bag itself light in weight and compact in size. For such a purpose, an inflator is also demanded for making its size compact. However, compacting the inflator results in high output and high temperature of the generated gas and the pressure applied to the air bag itself by the gas upon development tends to become high. Unfortunately, when the pressure upon development becomes high, there is a problem that gas leakage from base fabric or particularly from sewn part of base fabric of an air bag becomes high whereby the properties as an air bag cannot be satisfied. For solving this problem, various air bags have been proposed already (for example, see Patent Documents 1-4).
In the art of Patent Document 1, fineness and density of base fabric, and coating amount of synthetic resin, etc. are adjusted as a result of paying attention to gas leakage from stitches in a sewn part whereby air permeability of the sewn part under the presumed pressure at the time of development and of receiving the passenger is adjusted. However, the Patent Document 1 merely discloses that air permeability of the sewn part is affected by air permeability of the main body part consisting of the base fabric. The Patent Document 1 does not disclose a method for adjusting the air permeability of the sewn part itself.
Patent Document 2 discloses an air bag where auxiliary fabric is integrally sewn to a sewn part as a result of paying the attention to gas leakage and burst due to stitch opening of the sewn part. According to such an air bag, air permeability in the sewn part is reduced and, even in the base fabric of low fineness, suppression of burst upon development is possible. However, practical implementation of said air bag is difficult because of the disadvantages such as troublesomeness of the work, an increase in the mass of air bag and also an increase in cost due to an integral sewing of the auxiliary fabric.
Patent Document 3 discloses a base fabric for an air bag where three indexes of cover factor, tensile strength and edgecomb resistance are stipulated to specific ranges. Although the base fabric for an air bag achieves improvement in edgecomb resistance and reduction in slippage using a thread of relatively high fineness (not less than 400 dtex) and making the cover factor as high as possible, there is no investigation at all therein for a base fabric where the elongation of base fabric and the elongation of sewn part upon development are within predetermined ranges particularly because of high fineness of the base fabric.
Patent Document 4 discloses an air bag where a base fabric of very low fineness (110 to 185 dtex) is sewn with a precise pitch using sewing thread of low fineness. This air bag is light in weight and compact in size by decreasing the slippages but, due to the use of base fabric of low fineness, the air bag is not resistant to the gas of high output and high temperature generated from a compacted inflator.
Patent Documents 5 and 6 each discloses a base fabric for non-coat air bag using a fiber having a single thread in a rectangular cross section and a flat cross section, respectively. In the base fabric as such, flatness and surface unevenness of cross section of the single thread and also amount of oil in the spinning are stipulated to increase the friction coefficient between the fibers whereupon slidability of the thread surface is controlled and improvement in slippage of the sewn part is achieved. However, in those Patent Documents, there is no investigation at all for the base fabric where slippage of sewn part due to the specifying of the relation between elongations of “base fabric” and “sewing thread” upon development of an air bag or, particularly, the elongation of base fabric and the elongation of sewn part upon development are within predetermined ranges due to the high fineness of the base fabric.